Method for inserting a spinal implant

ABSTRACT

An artificial spinal implant is disclosed which when placed between two adjacent vertebrae directly participates and is incorporated in the ensuing fusion. Instrumentation and procedure is also disclosed.

BACKGROUND

The present invention relates to an artificial fusion implant to beplaced into the intervertebral space left after the removal of a damagedspinal disc.

The purpose of the present invention is to provide an implant to beplaced within the intervertebral disc space and provide for thepermanent elimination of all motion at that location. To do so, thedevice is space occupying within the disc space, rigid, self-stabilizingto resist dislodgement, stabilizing to the adjacent spinal vertebrae toeliminate local motion, and able to intrinsically participate in avertebra to vertebra bony fusion so as to assure the permanency of theresult.

At present, following the removal of a damaged disc, either bone ornothing is placed into the space left. Placing nothing in the spaceallows the space to collapse which nay result in damage to the nerves;or the space may fill with scar tissue and eventually lead to areherniation. The use of bone is less than optimal in that the boneobtained from the patient requires additional surgery and is of limitedavailability in its most useful form, and if obtained elsewhere, lacksliving bone cells, carries a significant risk of infection, and is alsolimited in supply as it is usually obtained from accident victims.Furthermore, regardless of the source of the bone, it is only marginalstructurally and lacks a means to either stabilize itself againstdislodgement, or to stabilize the adjacent vertebrae.

A review of related prior art will demonstrate the novelty of thepresent invention.

There have been an extensive number of attempts to develop an acceptabledisc prothesis (an artificial disc). Such devices by design would beused to replace a damaged disc and seek to restore the height of theinterspace and to restore the normal motion of that spinal joint. Nosuch device has been found that is medically acceptable. This group ofprosthetic or artificial disc replacements, seeking to preserve spinalmotion and so are different from the present invention, would include:

U.S. Pat. No. 3,867,728 STUBSTAD—describing a flexible disc implant.

U.S. Pat. No. 4,349,921 KUNTZ—describing a flexible disc replacementwith file like surface projections to discourage device dislocation.

U.S. Pat. No. 4,309,777 PATIL—describing a motion preserving implantwith spiked outer surfaces to resist dislocation and containing a seriesof springs to urge the vertebrae away from each other.

U.S. Pat. No. 3,875,595 FRONING—describing a motion preserving bladderlike disc replacement with two opposed stud-like projections to resistdislocation.

U.S. Pat. No. 2,372,622 FASSIO (FRENCH)—describing a emotion preservingimplant comprising complimentary opposed convex and concave surfaces.

In summary then, these devices resemble the present invention only inthat they are placed within the intervertebral space following theremoval of a damaged disc. In that they seek to preserve spinal motion,they are diametrically different from the present invention which seeksto permanently eliminate all motion at that spinal segment.

A second related area of prior art includes those devices utilized toreplace essentially wholly removed vertebra. Such removal is generallynecessitated by extensive vertebral fractures, or tumors, and is notassociated with the treatment of disc disease. While the presentinvention is to be placed within the disc space, these other vertebraldevices cannot be placed within the disc space as at least one vertebrahas already been removed such that there no longer remains a “discspace.” Furthermore, these devices are limited in that they seek toperform as temporary structural members mechanically replacing theremoved vertebra (not a removed disc), and do not intrinsicallyparticipate in supplying osteogenic material to achieve cross vertebraebony fusion. Therefore, again unlike the present invention whichprovides for a source of osteogenesis, use of this group of devices mustbe accompanied by a further surgery consisting of a bone fusionprocedure utilizing conventional technique. This group consisting ofvertebral struts rather than disc replacements would include thefollowing:

U.S. Pat. No. 4,553,273 WU—describing a turnbuckle like vertebral strut.

U.S. Pat. No. 4,401,112 REZAIAN—describing a turnbuckle like vertebralstrut with the addition of a long stabilizing staple that spans themissing vertebral body.

U.S. Pat. No. 4,554,914 KAPP—describing a large distractible spike thatelongates with a screw mechanism to span the gap left by the removal ofa entire vertebra and to serve as an anchor for acrylic cement which isthen used to replace the missing bone (vertebra).

U.S. Pat. No. 4,636,217 OGILVIE—describing a vertebral strut mechanismthat can be implanted after at least one vertebra has been removed andwhich device consists of a mechanism for causing the engagement ofscrews into the vertebra above and the vertebra below the one removed.

In summary then, this group of devices differs from the presentinvention in that they are vertebra replacements struts, do notintrinsically participate in the bony fusion, can only be inserted inthe limited circumstances where an entire vertebra has been removed fromthe anterior approach, and are not designed for, or intended to be usedfor the treatment of disc disease.

A third area of prior art related to the present invention includes alldevices designed to be applied to one of the surfaces of the spine. Suchdevices include all types of plates, struts, and rods which are attachedby hooks, wires and screws. These devices differ significantly from thepresent invention in that they are not inserted within the disc space,and furthermore do not intrinsically participate in supplying osteogenicmaterial for the fusion.

Therefore, with these devices where permanent spinal immobilization isdesired an additional surgery consisting of a spinal fusion performed byconventional means or the use of supplemental methylmethacrylate cementis required. Such devices applied to the spine, but not within the discspace, would include the following:

U.S. Pat. No. 4,604,995—STEPHENS—describing a “U” shaped metal rodattached to the posterior elements of the spine with wires to stabilizethe spine over a large number of segments.

U.S. Pat. No. 2,677,369—KNOWLES—describing a metal column device to beplaced posteriorly along the lumbar spine to be held in position by itsshape alone and to block pressure across the posterior portions of thespinal column by locking the spine in full flexion thereby shifting themaximum weight back onto the patient's own disc.

Other devices are simply variations on the use of rods (e.g. HarringtonLuque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer), plates andscrews (Steffee), or struts (Dunn, Knowles).

In summary, none of these devices are designed or can be used within thedisc space, do not replace a damaged disc, and do not intrinsicallyparticipate in the generation of a bony fusion.

Another area of related prior art to be considered is that of devicesdesigned to be placed within the vertebral interspace following theremoval of a damaged disc, and seeking to eliminate further motion atthat location.

Such a device is contained in U.S. Pat. No. 4,501,269 BAGBY—describingan implantable device and limited instrumentation. The method employedis as follows: a hole is bored transversely across the joint and then ahollow metal basket of larger diameter than the hole is then poundedinto the hole and then filled with the bone debris generated by thedrilling.

While the present invention (device, instrumentation, and method) mayappear to bear some superficial resemblance Lo the BAGBY invention, itis minimal, while the differences are many fold and highly significant.These differences include the following:

1. Safety

The present invention provides for a system of completely guardedinstrumentation so that all contiguous vital structures (e.g. largeblood vessels, neural structures) are absolutely protected. Saidinstrumentation also makes overpenetration by the drill impossible. Suchoverpenetration in the cervical spine, for example, would result in thetotal paralysis or death of the patient. In the thoracic spine, theresult would be complete paraplegia. In the lumbar spine, the resultwould be paraplegia or a life-threatening perforation of the aorta, venacava, or iliac vessels. The present invention is atraumatically screwedinto place while the BAGBY device, in contradistinction, is pounded intoposition. BAGBY describes that the implant is significantly larger insize than the hole drilled and must be pounded in. This is extremelydangerous and the pounding occurs directly over the spinal cord which isprecariously vulnerable to percussive injury. Furthermore, while it ispossible, for example in the lumbar spine, to insert the presentinvention away from the spinal cord and nerves, the BAGBY device mustalways be pounded directly towards the spinal cord.

Furthermore, since the BAGBY device is pounded into a smooth hole undergreat resistance, and lacking any specific design features to secure it,the device is highly susceptible to forceful ejection which would resultin great danger to the patient and a clinical failure. The presentinvention, in contradistinction, is securely screwed into place, andpossesses highly specialized locking threads to make accidentaldislodgement impossible. Because of the proximity of the spinal cord,spinal nerves, and blood vessels, any implant dislodgement as mightoccur with the BAGBY device might have catastrophic consequences.

2. Broad Applicability

The BAGBY device can only be inserted from the front of the vertebralcolumn, however, the present invention can be utilized in the cervical,thoracic, and lumbar spine, and can be inserted from behind(posteriorly) in the lumbar spine. This is of great importance in thatthe purpose of these devices is in the treatment of disc disease andprobably greater than 99 percent of all lumbar operations for thetreatment of disc disease are performed from behind where the presentinvention can easily be utilized, but the BAGBY device, as per hisdescription, cannot.

3. Disc Removal

The BAGBY invention requires the complete removal of the disc prior tothe drilling step, whereas the present invention eliminates thelaborious separate process of disc removal and efficiently removes thedisc and prepares the vertebral end plates in a single step.

4. Time Required

The present invention saves time over the BAGBY invention in that timeis not wasted laboring to remove the disc prior to initiating thefusion. Also, since with the present invention the procedure isperformed through a system of guarded instrumentation, time is notwasted constantly placing and replacing various soft tissue retractorsthroughout the procedure.

5. Implant Stability

Dislodgement of the implant would be a major source of device failure(an unsuccessful clinical result), and might result in patient paralysisor even death. As discussed, the BAGBY device lacks any specific meansof achieving stability and since it is pounded in against resistance toachieve vertebral distraction, it is susceptible to forcefuldislodgement by the tendency of the two distracted vertebrae, to returnto their original positions squeezing out the device. The presentinvention however is screwed into place. As there is no unscrewing forcepresent between the vertebrae and compression alone cannot dislodge theimplant, the implant is inherently stable by its design. Furthermore,the threads of the present invention are highly specialized in that theyare periodically interrupted such that the tail ends of each of the Labsso formed are blunted and twisted so as to resist accidental unscrewing.The removal of an implant with such “locking threads” requires the useof a special extractor included within the instrumentation. Thestability of the present invention is still further enhanced, again incontradistinction to the BAGBY device, by the presence of a “boneingrowth” surface texturing, which both increases the friction of thefit and allows for the direct growth of the vertebral bone into thecasing of the implant itself.

6. Spinal Stability

The present invention is not only self-stabilizing, it also providesstability to the adjacent vertebrae in at least three ways that theBAGBY device cannot. First, the BAGBY device is placed transverselyacross the joint in the center, leaving both vertebrae free to rock backand forth over this round barrel shaped axis, much like a board over abarrel, being used for a seesaw.

Secondly, as the BAGBY device lacks any specific design features toresist sliding, it may actually behave as a third body allowing thetranslation of the vertebrae relative to the device and to each other.

Thirdly, any device can only provide stability if it remains properlyseated. The present invention is inherently stable, and thereforeassures that it will stabilize the adjacent vertebrae; rather than, aswith the BAGBY device, where the instability of the spine to be treatedmay instead cause a dislocation of the implant, with further loss ofspinal stability.

7. The Collapse of the Interspace

While both the present invention and the BAGBY device can be fabricatedto withstand the compression forces within the interspace, theinterspace may nevertheless collapse under the superincumbent bodyweight as the implant settles into the vertebral bone. This is relatedto the load per unit area. Again the present invention is superior tothe BAGBY device in at least four ways. First, the present inventionoffers considerably greater surface area to distribute the load.Secondly, while the BAGBY device is placed centrally, the present deviceis placed bilaterally where the bone tends to be more cortical and muchstronger out towards the rim. Thirdly, the present invention supportsthe load achieving an “I” beam effect, whereas the BAGBY implant doesnot. Fourthly, it is not pressure alone that causes the collapse of thebone adjacent to the implant, but also bony erosion that is caused bythe motion under pressure of the implant against the bone. As discussedin item #6 above, the present invention alone is highly resistant tosuch motion, again diminishing the likelihood of erosion and interspacecollapse.

8. Bone Ingrowth Surface Texturing

The present invention has a surface treatment of known and conventionaltechnology to induce the growth of bone from the vertebrae directly intothe casing material of the implant itself. The BAGBY device has nosimilar feature.

9. Fusion Mass

The BAGBY invention calls for removing the disc and then drilling a holebetween the adjacent vertebrae. The bony debris so generated is then putinto tie device. The present invention takes a core of pure boneproducing marrow from the iliac crest, and then by use of a specialpress forcibly injects the device with an extremely dense compressedcore of that osteogenic material until the material itself virtuallyextrudes from every cell of the implant.

10. The Probability of Achieving Fusion

The fusion rate within the spine is known to be related directly to theamount of exposed vascular bone bed area, the quality and quantity ofthe fusion mass available, and the extent of the stabilization obtainedwith all other factors being held constant. It would then beanticipated, that the fusion rate would be superior with the presentinvention as compared to the BAGBY device, because of optimal implantstability (#5), optimal spinal stability (#6), bone ingrowth surfacetreatment (#8), superior fusion mass (#9), and the greater exposedvertebral bony surface area (#7).

The last area of prior art possibly related to the present invention andtherefore, to be considered related to “BONY INGROWTH”, and patentseither describe methods of producing materials and or materials ordevices to achieve the same. Such patents would include:

U.S. Pat. No. 4,636,526 (DORMAN), No. 4,634,720 (DORMAN), No. 4,542,539(ROWE), No. 4,405,319 (COSENTINO) No. 4,439,152 (SMALL), No. 4,168,326(BROEMER), No. 4,535,485 (ASHMAN), No. 3,987,499 (SCHARBACH), No.3,605,123 (HAHN), No. 4,655,777 (DUNN), No. 4,645,503 (LIN), No.4,547,390 (ASHMAN), No. 4,608,052 (VAN KAMPEN), No. 4,698,375 (DORMAN),No. 4,661,536 (DORMAN), No. 3,952,334 (BOKROS), No. 3,905,047 (LONG),No. 4,693,721 (DUCHEYNE), No. 4,070,514 (ENTHERLY).

However, while the present invention would utilize bone ingrowthtechnology, it would do so with conventional technology.

In summary then, the present invention, instrumentation, and method,alone provides for a one stage discectomy, fusion, and interbodyinternal spinal fixation; that being performed more quickly, withgreater safety, and more affectively, for all of the aforementionedreasons than is possible with any other known art.

BRIEF SUMMARY OF THE INVENTION

The present invention comprises a series of artificial implants, thepurpose of which is to participate in, and directly cause bone fusionacross an intervertebral space following the excision of a damaged disc.Such implants are structurally load bearing devices, stronger than bone,capable of withstanding the substantial forces generated within thespinal interspace. Such devices have a plurality of macro sized cellsand openings, which can be loaded with fusion promoting materials, suchas autogenous bone, for the purpose of materially influencing theadjacent vertebrae to form a bony bond to the implants and to eachother. The implant casing may be surface textured or otherwise treatedby any of a number of known technologies to achieve a “bone ingrowthsurface” to further enhance the stability of the implant and to expeditethe fusion.

Further, said devices are so configured and designed so as to promotetheir own stability within the vertebral interspace and to resist beingdislodged, and furthermore, to stabilize the adjacent spinal segments.

The apparatus for preparing the vertebrae for insertion of the implantis also disclosed, such instrumentation and method allowing for therapid and safe removal of the disc, preparation of the vertebrae,performance of the fusion, and internal stabilization of the spinalsegment.

DISCUSSION OF THE INSTRUMENTATION

The concept of performing various aspects of this surgery are notentirely new. Drills are frequently placed through hollow, tubularguards to protect the adjacent soft tissues. A set of instrumentsdeveloped by Ralph Cloward utilizes such a tubular drill guard on alarger scale, for the purpose of drilling into the cervical spine.However, this inventor is unaware of any set of instruments, system, orprocedure designed to allow the entire surgical procedure beyond theinitial exposure, to be performed blindly and with complete safetythrough a fixed sheath apparatus. Specific design features which combineto make this uniquely possible are as follows:

1. The availability of the specific implant.

2. The end of all the penetrating instrumentation is blunt faced.

3. All of the instruments have been stopped out at a predetermined depthto avoid overpenetration.

4. The design of the external sheath conforms to the spacial limitationsof the specific surgical site.

5. The design and use of a second or inner sheath allows for thedifference in size between the inside diameter of the outer sheath, andthe outside diameter of the drill itself. This difference beingnecessary to accommodate the sum of the distraction to be produced, andthe depth of the circumferential threading present on the implant.

6. A specially designed drill bit with a central shaft recess allows forthe safe collection of the drilling products, which can then be removedwithout disturbing the outer sheath by removing the drill bit and innersheath as a single unit.

7. A specially designed trephine for removing a core of bone slightlysmaller in diameter than the internal diameter of the implant cavityitself, however of a greater length.

8. A specially designed press for forcefully compressing and injectingthe long core of autogenous bone into the implant, such that it extrudesthrough the implant itself.

9. A specially designed driver extractor, which attaches to the implantand allows the implant to be either inserted or removed without itselfdissociating from the implant, except by the deliberate disengagement ofthe operator.

OBJECTS OF THE PRESENT INVENTION

It is an object of the present invention to provide an improved methodof performing a discectomy, a fusion, and an internal stabilization ofthe spine, and specifically, all three of the above simultaneously andas a single procedure.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine, which is both quicker and safer than ispossible by previous methods.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine, to provide for improved surgical spinalimplants.

It is another object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine, which provides for an improved system ofsurgical instrumentation to facilitate the performance of the combineddiscectomy, fusion, and internal spinal stabilization.

These and other objects of the present invention will be apparent fromreview of the following specifications and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the driver and sheath and itsorientation to a vertebral structure.

FIG. 1A is a perspective view of the driver member for the outer sheath.

FIG. 2 is a perspective view of the outer sheath being inserted into thevertebrae structure.

FIG. 3 is a perspective view of the outer sheath and inner sheathassembly, with the drill bit of the present invention.

FIG. 3A is a side sectional view of the collar and drill bit of FIG. 3.

FIG. 4 is a perspective view of a cylindrical implant and vertebraestructure.

FIG. 4A is a perspective view of one preferred embodiment of theimplant.

FIG. 4B is a cross sectional view of the implant of FIG. 4A.

FIG. 4C is the driving and insertion equipment for the implant of FIG.4A.

FIG. 4d is a side sectional view of the driver and implant betweenvertebrae.

FIG. 5 is a sectional view of the vertebrae structure, taken along lines5—5 of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1 a vertebrae structure comprising two vertebrae V anda disc D between the two vertebrae, is shown. A hollow tubular drillsleeve 10 has teeth 12 at its lower end. The sleeve 10 has an enlargeddiameter upper portion 14.

A driver 16, shown in FIG. 1A, consists of a solid tubular member 18 andan increased diameter head 20. The external diameter of the solidtubular member 18 is slightly smaller than the inside diameter of thehollow tubular drill sleeve 10 and has a length that is substantiallyshorter than the overall length of the hollow tubular drill sleeve 10.

The drill sleeve 10 is made of metal in order to be driven into thevertebrae V and be held in place by the teeth 12 of the drill sleeve 10.

Referring to FIG. 2 the drill sleeve 10 with the driver 16 installed isshown being driven into two vertebrae V on either side of a disc D byhammer H.

Referring to FIG. 3 and 3a the drill assembly is shown. In FIG. 3 thedrill sleeve 10 is illustrated in the two vertebrae V, stradding thedisc D.

The retaining sleeve 15 has an outside diameter slightly smaller thanthe inside diameter of the drill sleeve 10, and a length substantiallythe same length as the drill sleeve 10. The retaining sleeve 15 has acollar 17 at its upper end for engaging the top of the drill sleeve 10.

The drill 22 comprises an upper portion 24, a central recessed portion26 and a lower cutting drill portion 28. The upper 24 and lower portion28 of the drill 22 have the same outside diameter. The drill 24 has acollar 30 attached to the upper portion 24 of the drill 22.

The outside diameter of the drill 22 is slightly smaller than the insidediameter of the retaining sleeve 15. The length of the drill, from thecollar 30 to the end of the drill bit, is such that a predeterminedportion of the drill bit 22 extends beyond the end 29 of the sleeve whenfully inserted.

Referring to FIG. 4, a cylindrical embodiment of the present inventionis shown, one implant positioned in the opening in the vertebrae anddisc formed by the drill 22, and a second implant shown prior toimplantation.

The cylindrical implant 50 comprises a hollow tubular member which inthe preferred embodiment is made of an ASTM surgically implantablematerial, and preferably Titanium. The cylindrical implant 50 is closedat one end 52 and open at the other end 54. The outer cylindricalimplant 50 has a series of macro-sized openings 56 through the sidewalls of the cylindrical member 50. A series of external threads 53 areformed on the circumference of the cylindrical implant 50. The threads53 are locking threads having a series of interjections, the ends ofwhich are blunted and twisted so as to resist unscrewing.

The open end 54 of the cylindrical implant 50 has an internal thread 51for receiving a complementary cap 82 which has an external thread 58 forengaging the internal threads 51 of the cylindrical member 50. The cap52 has a hexagonal opening 59 for use with an allen wrench fortightening the cap. A driver engaging element 70 is located on the rearsurface 60 of the implant. The driver engaging element 70 comprises araised rectangular portion 63 and a central threaded opening 65, forengaging the driver apparatus, shown in FIG. 4c and FIG. 4d. The drivingequipment 100 comprises a central tubular rod 102 having a threadfitting into opening 65 in the implant. An enlarged knurled knob 106 isaffixed to the other end of the rod 102 for ease in turning. The centralrod 102 is enclosed within a hollow tubular member 108, having a narrowlower portion 110 and an increased diameter upper portion 112. At theend of the lower portion 110 is a attachment member 114, having agenerally rectangular depression 116 for complementing the driverengaging element 70 of the implant 50. A pair of handles 118 and 120extend perpendicular from the upper position 112 of the tubular member108 to assist in turning the driver 100.

An embodiment of the method of the present invention comprises the stepsof inserting a tubular member into the vertebrae, removing the disc anda portion of the adjacent vertebrae through the tubular member,inserting an implant into the vertebrae through the tubular member, andthen removing the tubular member.

The operation is performed in the following manner: (Example LumbarSpine Posterior Approach) A skin incision is made directly over oneinterspace to be operated on. The dissection is carried down along sideof the superspinous and intraspinous ligaments preserving thosestructures. A semi hemi laminotomy is performed at the upper level,removing sufficient bone to allow access into the interspace. Theligament flavum is removed and then the dural sac is protected byretracting it medially along with the traversing (inferior) nerve root.The superior nerve root or the root exiting beneath the pedicle at thelevel above is visualized and protected.

At this time the drill sleeve 10 is placed into the spinal canal withboth nerve roots directly inspected and protected. The drill sleeve 10is imbedded by teeth 12 spanning the disc space from the midline overand it is seated into the two vertebrae V across the disc D space byusing a driver 20. Once this is done, the driver 20 is removed and aretaining sleeve 15 is placed through the drill sleeve 10. Once seated,sleeve 10 provides absolute protection to the dural sac and nerve rootsas the remaining surgery is performed entirely through this sleeve.

The inner sleeve allows for the difference between the outside diameterof the drill 22 and the outside diameter of the threads 53 of thecylindrical implant 50. This then makes it possible to perform theentire operation through the end of the imbedded outer sleeve despitethe differences in diameter between the drill and the implant.

A drill 22 is then placed in the retaining sleeve 17. The drill 22 is ofsuch a length that it can not penetrate more then 28 millimeters beyondthe end of the drill sleeve 10. This, of course, could be varied andmade smaller for enhanced safety. However at the present time 27 to 28millimeters seems to be safe for probably 3 standard deviations of thepopulation. The drill 22 is attached to a power unit and the drillingtakes place.

The recessed central area between the reduced portion 26, allows for theaccumulation of the debris generated by the drilling. At this time,leaving the outer sleeve firmly embedded, the retaining sleeve 17 isremoved with the drill 22 as a single unit. All the vertebrae and discdebris that was generated during the drilling is contained within therecess and against the inside wall of the retaining sleeve 17 and cannot come out within the spinal canal. Once the retaining sleeve 17 anddrill 22 is out of the patient's operative field, all of the material sogenerated can be removed.

The next step is that a screw tap is put down through the drill sleeve10. The tap also has a collar on it that will automatically stop the tapfrom extending beyond 28 millimeters of penetration. The tap itself hasa blunt nose that would also avoid any perforation. The tap is thenremoved. The tap size has deliberately been selected so that it's innerroot diameter is 1.3 millimeters greater than the outside diameter ofthe drill 22. This insures that the interspace will be distracted by atleast that much once the implant is placed. The tap has its outsidediameter 1.2 millimeters greater than its root diameter. The tap isremoved and the space is now prepared to accept the cylindrical implant50.

The Implant 50 is prepared by utilizing the trephine, a hollow drill, toobtain a core of pure cancellous bone from the patients iliac crest ofslightly smaller diameter than the internal diameter of the implant butapproximately 6 mm longer. The implant 50 is placed in a press licedevice like an ammo loader and the bone graft measuring approximately 32millimeters is then compressed into the hollow body of the implant (26mm) so that the bone graft fills the opening 54 and extends through theopenings 56. The cap 60 is then screwed on to the implant 50 by use ofan alien driver/wrench and the device is ready for implantation.

The inserter/remover is such that it locks onto the implant, so that theimplant can be moved either clockwise or counter-clockwise, screwed orunscrewed. The implant itself has for its root diameter the same exactroot diameter as the tap which as already noted is already 1.3millimeters greater than the drill and has an outside diameter, 1.5millimeters greater than its root. This is also 0.3 millimeters greaterthan the threads cut by the tap so that in inserting the device it isactually cutting through previously uncut bone helping to insure that itlocks in firmly. The threads on the implant 50 are locking threads sothat it is easier to screw the device in than for it to be unscrewed.However, with sufficient torque it is possible to extract the device ifones so desires.

Once the implant has been seated it is able to be inserted only 28millimeters. Since the implant 50 is only 26 millimeters in length, thisvirtually guarantees that the implant 50 will be recessed into thevertebral bodies more than 2 millimeters and can not protrude into thespinal canal.

Similarly, the implants shown in FIG. 4b can be implanted. The implantin FIG. 4b is a modified solid, having extensive channelling throughout,and has no cap. A central opening 61 permits insertion of the bone graftmaterial into the interior of the implant.

These implants have a surface configuration such as to induce boneingrowth through the implant, and into the wall of the vertebrae ineffect inducing fusion from one vertebrae in joint to the other, therebyeventually making the implant itself superfluous as the bone would dothe work.

The implant itself, because of its being made of stronger material thanbone, would provide structural support to the two vertebrae whileawaiting bone ingrowth. Once the bone ingrowth occurred, however, theimplant would be firmly and permanently fixed in place.

As shown in FIG. 4, more than one implant is inserted into the discspace, thereby preventing the rocking motion that would result in thedifficulties referred to above in the discussion of the Bagby patent.

While the invention has been described with regards to the preferredembodiment, it is recognized that alternative embodiment may be devisedwhich would not depart from the present invention.

What I claim is:
 1. A method for preparing two adjacent vertebrae and adisc space therebetween to receive at least one implant, the methodconsisting essentially of the steps of: positioning one end of a hollowguard in contact with the adjacent vertebrae; removing, through saidhollow guard, bone from the adjacent vertebrae to form an opening acrossthe disc space and into a portion of each of the two adjacent vertebrae;and inserting said implant having a height greater than the normalheight of the disc space through said hollow guard and into the opening.2. The method of claim 1, wherein the positioning step includes the stepof engaging said one end of said hollow guard to the two adjacentvertebrae.
 3. The method of claim 1, wherein the removing step includesthe substep of milling the opening.
 4. The method of claim 1, whereinthe removing step includes the substep of drilling the opening.
 5. Themethod of claim 1, wherein the inserting step includes the substep ofscrewing said implant into the opening through said hollow guard.
 6. Themethod of claim 1, wherein the inserting step includes inserting saidimplant comprising a fusion-promoting substance.
 7. The method of claim1, further comprising the step of loading said implant withfusion-promoting material prior to the step of inserting.
 8. The methodof claim 1, wherein the positioning step includes the substep ofpositioning said guard having at least two projections extendingdistally from a distal end of said guard, said distal end adapted tocontact each of the vertebrae adjacent the disc space to be fused, saidat least two projections adapted to penetrate the disc space so as to belocated at least in part between the endplates of the vertebrae.
 9. Themethod of claim 1, wherein the positioning step includes the step ofpositioning one end of a guard in contact with the adjacent vertebrae ofthe posterior lumbar spine, said guard for providing protected access tothe spinal disc and the adjacent vertebrae to prepare across the spinaldisc and into the adjacent vertebrae the implantation space and toinsert into the implantation space said spinal implant through saidguard, said guard having a passage through which said spinal implantpasses into the implantation space in the spine, said guard having adistal end configured to be placed against the adjacent vertebrae, saiddistal end having a first portion adapted to be placed in contact withone of the adjacent vertebrae and a second portion adapted to be placedin contact with the other of the adjacent vertebrae, said first andsecond portions having a fixed height therebetween.
 10. The method ofclaim 1, wherein the method is performed without distracting theadjacent vertebrae between the removing step and the inserting step. 11.The method of claim 1, wherein the method is performed without removingthe hollow guard between the positioning step and the inserting step.12. A method for preparing two adjacent vertebrae and a disc spacetherebetween to receive at least one implant, the method consistingessentially of the steps of: positioning one end of a hollow guard incontact with the adjacent vertebrae; forming, through said hollow guard,an opening across the disc space and into a portion of each of the twoadjacent vertebrae, said opening having at least one arcuate portionextending into at least one of the adjacent vertebrae; and insertingsaid implant through said hollow guard and into the opening.
 13. Themethod of claim 12, wherein the positioning step includes the step ofengaging said one end of said hollow guard to the two adjacentvertebrae.
 14. The method of claim 12, wherein the forming step includesthe substep of milling the opening.
 15. The method of claim 12, whereinthe forming step includes the substep of drilling the opening.
 16. Themethod of claim 12, wherein the inserting step includes the substep ofscrewing said implant into the opening through said hollow guard. 17.The method of claim 12, wherein the inserting step includes insertingsaid implant comprising a fusion-promoting substance.
 18. The method ofclaim 12, further comprising the step of loading said implant withfusion-promoting material prior to the step of inserting.
 19. The methodof claim 12, wherein the positioning step includes the substep ofpositioning said guard having at least two projections extendingdistally from a distal end of said guard, said distal end adapted tocontact each of the vertebrae adjacent the disc space to be fused, saidat least two projections adapted to penetrate the disc space so as to belocated at least in part between the endplates of said vertebrae. 20.The method of claim 12, wherein the positioning step includes the stepof positioning one end of a guard in contact with the adjacent vertebraeof the posterior lumbar spine, said guard for providing protected accessto the spinal disc and the adjacent vertebrae to prepare across thespinal disc and into the adjacent vertebrae the implantation space andto insert into the implantation space said spinal implant through saidguard, said guard having a passage through which said spinal implantpasses into the implantation space in the spine, said guard having adistal end configured to be placed against the adjacent vertebrae, saiddistal end having a first portion adapted to be placed in contact withone of the adjacent vertebrae and a second portion adapted to be placedin contact with the other of the adjacent vertebrae, said first andsecond portions having a fixed height therebetween.
 21. The method ofclaim 12, wherein the method is performed without distracting theadjacent vertebrae between the forming step and the inserting step. 22.The method of claim 12, wherein the method is performed without removingthe hollow guard between the positioning step and the inserting step.23. A method for preparing two adjacent vertebrae and a disc spacetherebetween to receive at least one implant, the method comprising thesteps of: positioning one end of a hollow guard in contact with theadjacent vertebrae; removing, through said hollow guard, bone from theadjacent vertebrae to form an opening across the disc space and into aportion of each of the two adjacent vertebrae; and inserting saidimplant having a height greater than the normal height of the disc spacethrough said hollow guard and into the opening, the method beingperformed without distracting the adjacent vertebrae between theremoving step and the inserting step.
 24. The method of claim 23,further comprising the step of providing a spinal fusion implant of amaterial suitable for human implantation having a height along thelongitudinal axis of the spine greater than the height of theimplantation space comprising upper and lower portions adapted forplacement in contact each of the adjacent vertebrae when inserted in theimplantation space, each of said upper and lower portions having atleast one opening adapted for placement in contact with the adjacentvertebrae, respectively, said openings of said upper and lower portionsbeing in communication with one another and adapted for permitting forthe growth of bone from adjacent vertebra to adjacent vertebra throughsaid implant; and wherein the positioning step includes the step ofpositioning one end of a guard in contact with the adjacent vertebrae ofthe posterior lumbar spine, said guard for providing protected access tothe spinal disc and the adjacent vertebrae to prepare across the spinaldisc and into the adjacent vertebrae the implantation space and toinsert into the implantation space said spinal implant through saidguard, said guard having a passage through which said spinal implantpasses into the implantation space in the spine, said guard having adistal end configured to be placed against the adjacent vertebrae, saiddistal end having a first portion adapted to be placed in contact withone of the adjacent vertebrae and a second portion adapted to be placedin contact with the other of the adjacent vertebrae, said first andsecond portions having a fixed height therebetween.
 25. The method ofclaim 24, wherein the providing step includes the substep of providingsaid implant having at least a portion of said upper and lower portionsthat are arcuate along at least a portion of the length of said implant.26. The method of claim 25, wherein the providing step includes thesubstep of providing said implant further comprising a hollow betweensaid upper and lower arcuate portions of said implant for holding bonegrowth promoting material, said at least one opening of each of saidupper and lower portions of said implant are in communication with saidhollow to permit bone from adjacent vertebrae to grow through saidimplant.
 27. The method of claim 26, further comprising the step ofloading said hollow interior of said implant with fusion promotingsubstances.
 28. The method of claim 27, wherein the loading stepincludes the substep of loading said hollow interior of said implantwith bone.
 29. The method of claim 26, wherein the providing stepincludes the substep of providing said implant having at least one of aninsertion end and a trailing end that is open and adapted for loadingbone growth promoting substances into said hollow.
 30. The method ofclaim 29, further comprising the step of engaging an end cap for closingsaid open end of said implant.
 31. The method of claim 24, wherein theproviding step includes the substep of providing said implant that iscomprised at least in part of an implant material other than bone. 32.The method of claim 24, wherein the providing step includes the substepof providing said implant having said upper and lower portions having atleast one protrusion on said upper and lower portions for engaging theadjacent vertebrae.
 33. The method of claim 32, wherein said protrusionis at least a portion of a thread.
 34. The method of claim 24, whereinthe positioning step includes the substep of positioning said guardhaving projections projecting distally from said distal end forpenetrably engaging the posterior surface of each of the adjacentvertebrae.
 35. The method of claim 34, wherein said projections extendat least in part into the disc space.
 36. The method of claim 34,wherein the positioning step includes the substep of positioning saidguard further comprising a wall surrounding said passage, saidprojections being coextensive with said wall.
 37. The method of claim34, wherein the positioning step includes the substep of positioningsaid guard having at least two projections extending distally from saiddistal end of said guard, said distal end adapted to contact each of thevertebrae adjacent the disc space to be fused, said at least twoprojections adapted to penetrate the disc space so as to be located atleast in part between the endplates of said vertebrae.
 38. The method ofclaim 37, wherein said projections are at least in part co-linear withan outer perimeter of said guard.
 39. The method of claim 38, whereinsaid projections are teeth.
 40. The method of claim 37, wherein saidprojections are teeth being at least in part co-linear with an outerperimeter of said guard.
 41. The method of claim 24, wherein thepositioning step includes the substep of positioning said guard that isa tubular sleeve that is at least in part hollow.
 42. The method ofclaim 24, wherein the positioning step includes the substep ofpositioning said guard having a length defined by a distal portion and aproximal portion forming said length, said guard having a substantiallyuniform cross section along its distal portion.
 43. The method of claim24, wherein the positioning step includes the substep of positioningsaid guard having a substantially uniform cross section along itslength.
 44. The method of claim 43, wherein said distal end of saidguard has a cross sectional configuration substantially the same as saidsubstantially uniform cross section of said guard.
 45. The method ofclaim 24, wherein the positioning step includes the substep ofpositioning said guard having a distal end having a circumference thatis uninterrupted and constant.
 46. The method of claim 45, furthercomprising projections extending from said uninterrupted and constantcircumference of said distal end of said guard for engaging the spine.47. The method of claim 46, wherein said projections penetrate the discspace.
 48. The method of claim 24, wherein the positioning step includesthe substep of positioning said guard having a width measuredperpendicular to its height, the width of said guard when appropriatelyselected being less than one half of the width of the disc space. 49.The method of claim 24, further comprising the step of tapping theopening through said hollow guard.
 50. The method of claim 24, whereinthe inserting step includes the substep of screwing said implant intothe opening through said hollow guard.
 51. The method of claim 24,wherein the providing step includes the substep of providing saidimplant having at least one of an insertion end and a trailing end, saidtrailing end being adapted to cooperatively engage a driver forinserting said implant through said hollow guard and into the opening.52. The method of claim 23, wherein the positioning step includes thesubstep of positioning said guard having projections projecting distallyfrom a distal end for penetrably engaging the surface of each of theadjacent vertebrae.
 53. The method of claim 52, wherein said projectionsextend at least in part into the disc space.
 54. The method of claim 52,wherein the positioning step includes the substep of positioning saidguard further comprising a wall surrounding a passage, said projectionsbeing coextensive with said wall.
 55. The method of claim 52, whereinthe positioning step includes the substep of positioning said guardhaving at least two projections extending distally from a distal end ofsaid guard, said distal end adapted to contact each of the vertebraeadjacent the disc space to be fused, said at least two projectionsadapted to penetrate the disc space so as to be located at least in partbetween the endplates of the vertebrae.
 56. The method of claim 23,wherein the method is performed without removing the hollow guardbetween the positioning step and the inserting step.
 57. A method forpreparing two adjacent vertebrae and a disc space therebetween toreceive at least one implant, the method comprising the steps of:positioning one end of a hollow guard in contact with the adjacentvertebrae; forming, through said hollow guard, an opening across thedisc space and into a portion of each of the two adjacent vertebrae,said opening having at least one arcuate portion extending into at leastone of the adjacent vertebrae; and inserting said implant through saidhollow guard and into the opening, the method being performed withoutdistracting the adjacent vertebrae between the removing step and theinserting step.
 58. The method of claim 57, wherein the positioning stepincludes the substep of positioning said guard having at least twoprojections extending distally from a distal end of said guard, saiddistal end adapted to contact each of the vertebrae adjacent the discspace to be fused, said at least two projections adapted to penetratethe disc space so as to be located at least in part between theendplates of said vertebrae.
 59. The method of claim 57, wherein thepositioning step includes the step of positioning one end of said guardin contact with the adjacent vertebrae of the posterior lumbar spine,said guard for providing protected access to the spinal disc and theadjacent vertebrae to prepare across the spinal disc and into theadjacent vertebrae the implantation space and to insert into theimplantation space said spinal implant through said guard, said guardhaving a passage through which said spinal implant passes into theimplantation space in the spine, said guard having a distal endconfigured to be placed against the adjacent vertebrae, said distal endhaving a first portion adapted to be placed in contact with one of theadjacent vertebrae and a second portion adapted to be placed in contactwith the other of the adjacent vertebrae, said first and second portionshaving a fixed height therebetween.
 60. The method of claim 57, whereinthe method is performed without removing said hollow guard between thepositioning step and the inserting step.
 61. A method for preparing twoadjacent vertebrae and a disc space therebetween to receive at least oneimplant, the method comprising the steps of: positioning one end of ahollow guard in contact with the adjacent vertebrae; removing, throughsaid hollow guard, bone from the adjacent vertebrae to form an openingacross the disc space and into a portion of each of the two adjacentvertebrae; and screwing said implant having an external thread and aheight greater than the normal height of the disc space through saidhollow guard and into the opening.
 62. The method of claim 61, whereinthe positioning step includes the substep of positioning said guardhaving at least two projections extending distally from a distal end ofsaid guard, said distal end adapted to contact each of the vertebraeadjacent the disc space to be fused, said at least two projectionsadapted to penetrate the disc space so as to be located at least in partbetween the endplates of the vertebrae.
 63. The method of claim 62,wherein the positioning step includes the step of positioning one end ofsaid guard in contact with the adjacent vertebrae of the posteriorlumbar spine, said guard for providing protected access to the spinaldisc and the adjacent vertebrae to prepare across the spinal disc andinto the adjacent vertebrae the implantation space and to insert intothe implantation space said spinal implant through said guard, saidguard having a passage through which said spinal implant passes into theimplantation space in the spine, said guard having a distal endconfigured to be placed against the adjacent vertebrae, said distal endhaving a first portion adapted to be placed in contact with one of theadjacent vertebrae and a second portion adapted to be placed in contactwith the other of the adjacent vertebrae, said first and second portionshaving a fixed height therebetween.
 64. The method of claim 61, whereinthe method is performed without distracting the adjacent vertebraebetween the removing step and the inserting step.
 65. The method ofclaim 61, wherein the method is performed without removing said guardbetween the positioning step and the screwing step.
 66. A method forpreparing two adjacent vertebrae and a disc space therebetween toreceive an implant, the method comprising the steps of: anchoring oneend of a substantially tubular member against the two adjacent vertebraeso that at least a portion of said one end of said tubular memberextends at least in part between the adjacent vertebrae and into thedisc space; forming through said tubular member, an opening across thedisc space and into a portion of each of the two adjacent vertebrae;tapping the opening to provide threads over at least a portion of thevertebrae; and screwing said implant through said tubular member andinto the opening.
 67. The method of claim 66, further comprising thestep of providing a spinal fusion implant of a material suitable forhuman implantation having a height along the longitudinal axis of thespine greater than the height of the implantation space, comprisingupper and lower portions adapted for placement in contact with each ofthe adjacent vertebrae when inserted in the implantation space, each ofsaid upper and lower portions having at least one opening adapted forplacement in contact with the adjacent vertebrae, respectively, saidopenings of said upper and lower portions being in communication withone another and adapted for permitting for the growth of bone fromadjacent vertebra to adjacent vertebra through said implant; and whereinthe anchoring step includes the step of anchoring one end of saidtubular member in contact with the adjacent vertebrae of the posteriorlumbar spine, said tubular member for providing protected access to thespinal disc and the adjacent vertebrae to prepare across the spinal discand into the adjacent vertebrae the implantation space and to insertinto the implantation space said spinal implant through said tubularmember, said tubular member having a passage through which said spinalimplant passes into the implantation space in the spine, said tubularmember having a distal end configured to be placed against the adjacentvertebrae, said distal end having a first portion adapted to be placedin contact with one of the adjacent vertebrae and a second portionadapted to be placed in contact with the other of the adjacentvertebrae, said first and second portions having a fixed heighttherebetween.
 68. The method of claim 67, wherein the providing stepincludes the substep of providing said implant having at least a portionof said upper and lower portions that are arcuate along at least aportion of the length of said implant.
 69. The method of claim 68,wherein the providing step includes the substep of providing saidimplant further comprising a hollow between said upper and lower arcuateportions of said implant for holding bone growth promoting material,said at least one opening of each of said upper and lower surfaces ofsaid implant are in communication with said hollow to permit bone fromadjacent vertebrae to grow through said implant.
 70. The method of claim69, further comprising the step of loading said hollow interior of saidimplant with fusion promoting substances.
 71. The method of claim 70,wherein the loading step includes the substep of loading said hollowinterior of said implant with bone.
 72. The method of claim 71, whereinthe anchoring step includes the substep of anchoring said tubular memberthat is a tubular sleeve that is at least in part hollow.
 73. The methodof claim 72, wherein the providing step includes the substep ofproviding said implant having at least one of an insertion end and atrailing end that is open and adapted for loading bone growth promotingsubstances into said hollow.
 74. The method of claim 73, furthercomprising the step of engaging an end cap for closing said open end ofsaid implant.
 75. The method of claim 67, wherein the providing stepincludes the substep of providing said implant that is comprised atleast in part of an implant material other than bone.
 76. The method ofclaim 67, wherein the providing step includes the substep of providingsaid implant having said upper and lower portions having at least oneprotrusion on said upper and lower portions for engaging the adjacentvertebrae.
 77. The method of claim 76, wherein said protrusion is atleast a portion of a thread.
 78. The method of claim 67, wherein theanchoring step includes the substep of anchoring said tubular memberhaving projections projecting distally from said distal end forpenetrably engaging the posterior surface of each of the adjacentvertebrae.
 79. The method of claim 78, wherein said projections extendat least in part into the disc space.
 80. The method of claim 78,wherein the anchoring step includes the substep of anchoring saidtubular member further comprising a wall surrounding said passage, saidprojections being coextensive with said wall.
 81. The method of claim78, wherein the anchoring step includes the substep of anchoring saidtubular member having at least two projections extending distally fromsaid distal end of said tubular member, said distal end adapted tocontact each of the vertebrae adjacent the disc space to be fused, saidat least two projections adapted to penetrate the disc space so as to belocated at least in part between the endplates of the vertebrae.
 82. Themethod of claim 81, wherein said projections are at least in partco-linear with an outer perimeter of said tubular member.
 83. The methodof claim 82, wherein said projections are teeth.
 84. The method of claim81, wherein said projections are teeth being at least in part co-linearwith an outer perimeter of said tubular member.
 85. The method of claim67, wherein the anchoring step includes the substep of anchoring saidtubular member having a length defined by a distal portion and aproximal portion forming said length, said tubular member having asubstantially uniform cross section along its distal portion.
 86. Themethod of claim 67, wherein the anchoring step includes the substep ofanchoring said tubular member having a substantially uniform crosssection along its length.
 87. The method of claim 86, wherein saiddistal end of said tubular member has a cross sectional configurationsubstantially the same as said substantially uniform cross-section ofsaid tubular member.
 88. The method of claim 67, wherein the anchoringstep includes the substep of anchoring said tubular member having adistal end having a circumference that is uninterrupted and constant.89. The method of claim 88, further comprising projections extendingfrom said uninterrupted and constant circumference of said distal end ofsaid tubular member for engaging the spine.
 90. The method of claim 89,wherein said projections penetrate the disc space.
 91. The method ofclaim 67, wherein the anchoring step includes the substep of anchoringsaid tubular member having a width measured perpendicular to its height,the width of said tubular member when appropriately selected being lessthan one half of the width of the disc space.
 92. The method of claim67, further comprising the step of tapping the opening through saidhollow tubular member.
 93. The method of claim 67, wherein the providingstep includes the substep of providing said implant having at least oneof an insertion end and a trailing end, said trailing end being adaptedto cooperatively engage a driver for inserting said implant through saidhollow tubular member and into the opening.
 94. The method of claim 66,wherein the method is performed without removing said tubular memberbetween the anchoring step and the screwing step.
 95. A method forpreparing two adjacent vertebrae and a disc space therebetween toreceive at least one implant, the method comprising the steps of:providing an implant of a material suitable for human implantation;positioning one end of a tubular member in contact with the adjacentvertebrae; forming, through said tubular member, an opening across thedisc space and into a portion of each of the two adjacent vertebrae; andinserting said implant through said tubular member and into the opening.96. The method of claim 95, wherein the positioning step includes thesubstep of positioning said guard having at least two projectionsextending distally from a distal end of said guard, said distal endadapted to contact each of the vertebrae adjacent the disc space to befused, said at least two projections adapted to penetrate the disc spaceso as to be located at least in part between the endplates of saidvertebrae.
 97. The method of claim 95, wherein the positioning stepincludes the step of positioning one end of a guard in contact with theadjacent vertebrae of the posterior lumbar spine, said guard forproviding protected access to the spinal disc and the adjacent vertebraeto prepare across the spinal disc and into the adjacent vertebrae theimplantation space and to insert into the implantation space said spinalimplant through said guard, said guard having a passage through whichsaid spinal implant passes into the implantation space in the spine,said guard having a distal end configured to be placed against theadjacent vertebrae, said distal end having a first portion adapted to beplaced in contact with one of the adjacent vertebrae and a secondportion adapted to be placed in contact with the other of the adjacentvertebrae, said first and second portions having a fixed heighttherebetween.
 98. The method of claim 95, wherein the method isperformed without distracting the adjacent vertebrae between the formingstep and the inserting step.
 99. The method of claim 95, wherein themethod is performed without removing said tubular member between thepositioning step and the inserting step.